Increasingly, commercially valuable information is being created and stored in "digital" form. For example, music, photographs and video can all be stored and transmitted as a series of numbers, such as 1's and 0's. Digital techniques let the original information be recreated in a very accurate manner. Unfortunately, digital techniques also let the information be easily copied without the owner's permission.
Digital watermarks exist at a convergence point where creators and publishers of digitized multimedia content demand local, secure identification and authentication of content. Because piracy discourages the distribution of valuable digital information, establishing responsibility for copies and derivative copies of such works is important. The goal of a digital watermark system is to insert a given information signal or signals in such a manner as to leave little or no artifacts, with one standard being perceptibility, in the underlying content signal, while maximizing its encoding level and "location sensitivity" in the signal to force damage to the content signal when removal is attempted. In considering the various forms of multimedia content, whether "master," stereo, National Television Standards Committee (NTSC) video, audio tape or compact disc, tolerance of quality will vary with individuals and affect the underlying commercial and aesthetic value of the content. It is desirable to tie copyrights, ownership rights, purchaser information or some combination of these and related data into the content in such a manner that the content undergoes damage, and therefore reduction of its value, with subsequent unauthorized distribution, commercial or otherwise. Digital watermarks address many of these concerns and research in the field has provided a rich basis for extremely robust and secure implementations.
Of particular concern is the balance between the value of a digitized "piece" of content and the cost of providing worthwhile "protection" of that content. In a parallel to real world economic behavior, the perceived security of a commercial bank does not cause people to immediately deposit cash because of the expense and time required to perform a bank deposit. For most individuals, possession of a US$100 bill does not require any protection beyond putting it into a wallet. The existence of the World Wide Web, or "Web," does not implicitly indicate that value has been created for media which can be digitized, such as audio, still images and other media. The Web is simply a medium for information exchange, not a determinant for the commercial value of content. The Web's use to exchange media does, however, provide information that helps determine this value, which is why responsibility over digitized content is desirable. Note that digital watermarks are a tool in this process, but they no not replace other mechanisms for establishing more public issues of ownership, such as copyrights. Digital watermarks, for example, do not replace the "historical average" approach to value content. That is, a market of individuals willing to make a purchase based solely on the perceived value of the content. By way of example, a picture distributed over the Internet, or any other electronic exchange, does not necessarily increase the underlying value of the picture, but the opportunity to reach a greater audience by this form of "broadcast" may be a desirable mechanism to create "potentially" greater market-based valuations. That decision rests solely with the rights holder in question.
Indeed, in many cases, depending on the time value of the content, value may actually be reduced if access is not properly controlled. With a magazine sold on a monthly basis, it is difficult to assess the value of pictures in the magazine beyond the time the magazine is sold. Compact disc valuations similarly have time-based variables, as well as tangible variables such as packaging versus the package-less electronic exchange of the digitized audio signals. The Internet only provides a means to more quickly reach consumers and does not replace the otherwise "market-based" value. Digital watermarks, properly implemented, add a necessary layer of ownership determination which will greatly assist in determining and assessing value when they are "provably secure." The present invention improves digital watermarking technology while offering a means to properly "tamper proof" digitized content in a manner analogous to methods for establishing authenticity of real world goods.
A general weakness in digital watermark technology relates directly to the way watermarks are implemented. Too many approaches leave detection and decode control with the implementing party of the digital watermark, not the creator of the work to be protected. This fundamental aspect of various watermark technologies removes proper economic incentives for improvement of the technology when third parties successfully exploit the implementation. One specific form of exploitation obscures subsequent watermark detection. Others regard successful over encoding using the same watermarking process at a subsequent time.
A set of secure digital watermark implementations address this fundamental control issue, forming the basis of "key-based" approaches. These are covered by the following patents and pending applications, the entire disclosures of which are hereby incorporated by reference: U.S. Pat. No. 5,613,004 entitled "Steganographic Method and Device" and its derivative U.S. patent application Ser. No. 08/775,216, U.S. patent application Ser. No. 08/587,944 entitled "Human Assisted Random Key Generation and Application for Digital Watermark System," U.S. patent application Ser. No. 08/587,943 entitled "Method for Stega-Cipher Protection of Computer Code," U.S. patent application Ser. No. 08/677,435 entitled "Optimization Methods for the Insertion, Protection, and Detection of Digital Watermarks in Digitized Data," and U.S. patent application Ser. No. 08/772,222 entitled "Z-Transform Implementation of Digital Watermarks." Public key crypto-systems are described in U.S. Pat. Nos. 4,200,770, 4,218,582, 4,405,829 and 4,424,414, the entire disclosures of which are also hereby incorporated by reference.
By way of improving these digital watermark security methods, utilization of multiple transforms, manipulation of signal characteristics and the requisite relationship to the mask set or "key" used for encoding and decoding operations are envisioned, as are optimized combinations of these methods. While encoding a watermark may ultimately differ only slightly in terms of the transforms used in the encoding algorithm, the greater issues of an open, distributed architecture requires more robust approaches to survive attempts at erasure, or even means for making detection of the watermark impossible. These "attacks," when computationally compared, may be diametrically related. For instance, cropping and scaling differ in signal processing orientation, and can result in the weakening of a particular watermarking approach but not all watermarking approaches.
Currently available approaches that encode using either a block-based or entire data set transform necessarily encode data in either the spatial or frequency domains, but never both domains. A simultaneous crop and scale affects the spatial and frequency domains enough to obscure most available watermark systems. The ability to survive multiple manipulations is an obvious benefit to those seeking to ensure the security of their watermarked media. The present invention seeks to improve on key-based approaches to watermarking previously disclosed, while offering greater control of the subsequently watermarked content to rights owners and content creators.
Many currently available still image watermarking applications are fundamentally different from the key-based implementations. Such products include products offered by Digimarc and Signum, which seek to provide a robust watermark by encoding watermark messages that rely entirely on comparisons with the original image for decode operations. The subsequent result of the transform, a discrete cosine transform performed in blocks, is digital signed. The embedded watermarks lack any relationship to the perceptual qualities of the image, making inverse application of the publicly available decoders a very good first line of attack. Similarly, the encoding process may be applied by third parties, as demonstrated by some robustness tests, using one process to encode over the result of an image watermarked with another process. Nonrepudiation of the watermark is not possible, because Digimarc and Signum act as the repository of all registrations of the image's ownership.
Another line of attack is a low pass filter that removes some of the high frequency noise that has been added, making error-free detection difficult or impossible. Finally, many tests of a simple JPEG transform indicate the watermarks may not survive as JPEG is based on the same transforms as the encoding transforms used by the watermarking process. Other notable implementations, such as that offered by Signafy (developed by NEC researchers), appear to encode watermark messages by performing a transform of the entire image. The goal of this process is to more consistently identify "candidate" watermark bits or regions of the image to encode in perceptually significant regions of the signal. Even so, Signafy relies on the original unwatermarked image to accomplish decoding.
All of these methods still rely on the original unwatermarked image to ensure relatively error-free detection of the watermarks. The steganographic method seeks to provide watermark security without an original unwatermarked copy of the media for decode operations, as well as providing users cryptographic security with ciphered symmetric keys. That is, the same key is used for encode and decode operations. Public key pairs, where each user has a public/private key pair to perform asymmetric encode and decode operations, can also be used. Discussions of public key encryption and the benefits related to encryption are well documented. The growing availability of a public key infrastructure also indicates recognition of provable security. With such key-based implementations of watermarking, security can be off-loaded to the key, providing for a layered approach to security and authentication of the watermark message as well as the watermarked content.
It is known that attacks on the survivability of other implementations are readily available. Interesting network-based attacks on the watermark message are also known which fool the central registration server into assuming an image is owned by someone other than the registered owner. This also substantiates the concern that centralized watermarking technologies are not robust enough to provide proper assurances as to the ownership of a given digitized copy of an multimedia work.
Because the computational requirements of performing multiple transforms may not be prohibitive for certain media types, such as still images and audio, the present invention seeks to provide a means to securely watermark media without the need for an original unwatermarked copy to perform decoding. These transforms may be performed in a manner not plainly evident to observers or the owner of the content, who may assume the watermark is still detectable. Additionally, where a particular media type is commonly compressed (JPEG, MPEG, etc.), multiple transforms may be used to properly set the mask sets, prior to the watermarking process, to alert a user to survivability prior to the release of a watermarked, and thus perceived, "safe" copy to unknown parties. The result of the present invention is a more realistic approach to watermarking taking the media type, as well as the provable security of the keys into consideration. A more trusted model for electronic commerce is therefore possible. The creation of an optimized "envelope" for insertion of watermarks to establish secured responsibility for digitally-sampled content provides the basis of much watermark security but is also a complementary goal of the present invention. The predetermined or random key that is generated is not only an essential map to access the hidden information signal, but is also the a subset of the original signal making direct comparisons with the original signal unnecessary. This increases the overall security of the digital watermark.
Survival of simultaneous cropping and scaling is a difficult task with image and audio watermarking, where such transformations are common with the inadvertent use of images and audio, and with intentional attacks on the watermark. The corresponding effects in audio are far more obvious, although watermarks which are strictly "frequency-based," such as variations of spread spectrum, suffer from alignment issues in audio samples which have been "cropped," or clipped from the original length of the piece. Scaling is far more noticeable to the human auditory system, though slight changes may affect frequency-only-type watermarks while not being apparent to a consumer. The far greater threat to available audio watermark applications, most of which are variations of frequency-based embedded signaling, are generally time-based transformations, including time-based compression and expansion of the audio signal. Signafy is an example of spread spectrum-based watermarking, as are applications by Solana Technology, CRL, BBN, MIT, etc. "Spatial domain" approaches are more appropriate designations for the technologies deployed by Digimarc, Signum, ARIS, Arbitron, etc. Interestingly, a time-based approached when considered for images is basically a "spatial-based" approach. The pixels are "convolutional." The difference being that the "spread spectrum-ed" area of the frequencies is "too" well-defined and thus susceptible to over-encoding of random noise at the same sub-bands as that of the embedded signal.
Giovanni uses a block-based approach for the actual watermark. However, it is accompanied by image-recognition capable of restoring a scaled image to its original scale. This "de-scaling" is applied before the image is decoded. Other systems used a "differencing" of the original image with the watermarked image to "de-scale." It is clear that de-scaling is inherently important to the survival of any image, audio or video watermark. What is not clear is that the differencing operation is acceptable from a security standpoint. Moreover, differencing that must be carried out by the watermarking "authority," instead of the user or creator of the image, causes the rights owner to lose control over the original unwatermarked content. Aside from utilizing the mask set within the encoding/decoding key/key pair, the original signal must be used. The original is necessary to perform detection and decoding, although with the attacks described above it is not possible to clearly establish ownership over the watermarked content.
In view of the foregoing, it can be appreciated that a substantial need exists for multiple transform utilization and applications for secure digital watermarking that solve the problems discussed above.